fix(tui): F-006 flatten multi-line paste in input render

A multi-line paste put raw newlines into the single-line input slot, overflowing
the fixed-height box and leaving a smeared/stale region. flattenForDisplay collapses
newlines (↵) and tabs for rendering only; the inputBuffer keeps real characters for
submission.

apps/tui-go/internal/app/input_test.go

@@ -0,0 +1,26 @@
+package app
+
+import (
+	"strings"
+	"testing"
+)
+
+// F-006: a multi-line paste must collapse to a single display line so the fixed-height
+// input box can't overflow and leave a smeared/stale region.
+func TestFlattenForDisplay(t *testing.T) {
+	cases := map[string]string{
+		"plain text":         "plain text",
+		"line one\nline two": "line one↵ line two",
+		"crlf\r\nhere":       "crlf↵ here",
+		"tab\tseparated":     "tab separated",
+		"a\nb\nc":            "a↵ b↵ c",
+	}
+	for in, want := range cases {
+		if got := flattenForDisplay(in); got != want {
+			t.Errorf("flattenForDisplay(%q) = %q, want %q", in, got, want)
+		}
+		if strings.ContainsAny(flattenForDisplay(in), "\n\r\t") {
+			t.Errorf("flattenForDisplay(%q) still contains a control char", in)
+		}
+	}
+}